State Key Laboratory of Fine Chemicals, Dalian University of Technology , Dalian 116024, China.
Research Institute of Chemical Engineering, Dalian University of Technology , Dalian 116024, China.
J Phys Chem B. 2017 Apr 13;121(14):3087-3098. doi: 10.1021/acs.jpcb.7b00231. Epub 2017 Mar 30.
1-Ethyl-3-methylimidazolium dimethylphosphate ([Emim][Dmp])-water binary solution is one of the promising new working-pairs for absorption heat pump and absorption chillers, which are widely used to recover industrial waste heat. In the absorption process, the mass and heat transfer at the interface greatly depend on interface microscopic structure. Therefore, in order to understand the absorption process, it is very important to study the interface microscopic structure. The liquid-vapor interface properties, as well as the orientation of [Emim], [Dmp], water at the interface and its aqueous solution with different water mole fraction, were studied using classical all-atom force field by molecular dynamic simulations. The simulated bulk mass density fitted by hyperbolic tangent function for each system was in good agreement with the experiment data, with the relative deviation between simulated and experimental value within 2%. The simulated results indicate that anion is always distributed at the outmost layer of the interface, followed by cation and water molecule. In [Emim][Dmp], the tilt angle of imidazolium rings to the surface normal is in the range of 0° < θ < 12°; for most cation, their ethyl and methyl tilted toward gas phase and bulk, respectively, but for a few cation, their ethyl and the methyl take the opposite orientation. For anion, one methyl prefers to turn toward gas phase and another methyl (PC vector from P atom to C atom) lie nearly parallel to the surface, while one PO vector (from P atom to O atom) turns toward liquid bulk and another PO vector is nearly parallel to the surface. In aqueous solution of [Emim][Dmp], the tilt angle of the imidazolium ring to the surface normal becomes larger (0° < θ < 37°) at the interface, but almost all ethyl intend to tilt toward gas phase and the methyl tilt toward liquid bulk compared with pure [Emim][Dmp]. Two methyl in anion prefer to turn toward gas phase and its two PO vectors toward liquid bulk. This orientation indicates that pure [Emim][Dmp] absorb water in gas phase more easily than [Emim][Dmp]+HO system does. Water molecules are distributed in the inner layer of the interface with two OH vectors (from O atom to H atom) tilting toward external surface.
1-乙基-3-甲基咪唑二甲磷酸酯([Emim][Dmp])-水二元溶液是吸收式热泵和吸收式制冷机中很有前途的新型工作对之一，被广泛应用于回收工业余热。在吸收过程中，界面处的质量和热量传递很大程度上取决于界面的微观结构。因此，为了理解吸收过程，研究界面的微观结构是非常重要的。通过分子动力学模拟，使用经典的全原子力场研究了气液界面的性质，以及[Emim]、[Dmp]、水在界面上的取向，以及不同水摩尔分数的水溶液。对于每个体系，通过双曲正切函数拟合的模拟体相密度与实验数据吻合良好，模拟值与实验值的相对偏差在 2%以内。模拟结果表明，阴离子总是分布在界面的最外层，其次是阳离子和水分子。在[Emim][Dmp]中，咪唑环相对于表面法线的倾斜角在 0°<θ<12°范围内；对于大多数阳离子，它们的乙基和甲基分别向气相和体相倾斜，但对于少数阳离子，它们的乙基和甲基则相反。对于阴离子，一个甲基倾向于转向气相，另一个甲基（从 P 原子到 C 原子的 PC 矢量）几乎与表面平行，而一个 PO 矢量（从 P 原子到 O 原子）转向液相，另一个 PO 矢量几乎与表面平行。在[Emim][Dmp]的水溶液中，界面处咪唑环相对于表面法线的倾斜角变大（0°<θ<37°），但与纯[Emim][Dmp]相比，几乎所有的乙基都倾向于转向气相，甲基倾向于转向液相。阴离子中的两个甲基倾向于转向气相，其两个 PO 矢量倾向于液相。这种取向表明，与[Emim][Dmp]+HO 体系相比，纯[Emim][Dmp]更容易在气相中吸收水。水分子分布在界面的内层，两个 OH 矢量（从 O 原子到 H 原子）向外部表面倾斜。
